Passive voice is to change the doer into object and the object into the doer.

To change active voice into passive or passivating an active voice, you should :

1. Determine the Tense being used.
2. Find the /be/, based on the tense.
3. exchange the position of the doer and the object.

For examples:

The English teacher often punishes me.

Indicators found are:

The doer = /The English Teacher/
Object = /me/
Tense being used = /Present Simple/. Look at the determiner of present simple, that is, /punishes/.

Now's time to change it!

Take notice that there are two elements added in the passive voice, they are:

1. Be /am/
2. By /Particle or preposition/

When the object is changed into the doer or the subject, /be/ /am/ must be used. Why /am/ ? . It is because the subject being used is in present simple or depends on the tense being used. In the sentence above the tense being used is in Present simple by looking at the determiner (suffix /-es/ in verb /punish/.

Use /by/ when there is an exact or clear object.

Study this following example:

The rice is planted.

The sentence above doesn't have an clear object. Why not?.
Just read the context carefully!. It is said that rice, generally, is planted by farmer. It is impossible if the rice is planted by engineer even though an engineer also plants it. But, when he or she plants the rice, we will call him or her "Farmer". Is that right?. (laughing ,.....).

Because the sentence has general meaning. I mean all people has already understood that who plants the rice must be a farmer, then the object /a farmer/ can be omitted or unusable. For this, the particle /by/ is not used.